1. Field of the Invention
The present invention relates to an ionization apparatus, more particularly relates to an ionization apparatus using ion attachment ionization and suitable for mass spectrometry of a target gas including a gaseous state organic substance, chemically active substance, or highly adsorptive substance.
2. Description of the Related Art
As for the mass spectrometry of various types of gases there are conventional mass spectrometry methods of ionizing a target gas to make ions, then separating and analyzing the ions by mass using one or both of an electric field and a magnetic field. There are various methods for ionization of the target gas. Among these, the ion attachment ionization method has the feature of a lower excess energy at the time of ionization than other methods of ionization. In this ion attachment ionization method, the target gas is defined as a gas being a target substance for analysis, while a sample gas is defined as a gas made by attaching metal ions to the target gas, which is measured by a mass spectrometer. The ion attachment ionization method enables measurement even when the target gas is comprised of gaseous molecules including weak bonds among atoms. The ion attachment ionization method has the advantage of a broader range of types of gas which can be measured than electron impact ionization, chemical ionization, or other methods.
The ion attachment ionization method enables for example alkali metal ions or other metal ions to be directly attached to gaseous molecules. The procedure and apparatus for this ionization method is for example disclosed in Japanese Unexamined patent Publication (Kokai) No. 6-11485 and Japanese Examined patent Publication (Kokoku) No. 7-48371.
Next, a representative conventional ionization apparatus for ion attachment mass spectrometry will be explained with reference to FIG. 12 and FIG. 13. FIG. 12 is a perspective view of an ionization apparatus showing part of the internal structure, while FIG. 13 is a schematic longitudinal sectional view of the same.
A conventional ionization apparatus used for an ion attachment mass spectrometry apparatus is comprised for example of a closed end cylindrically shaped hollow vessel 101 connected to a mass spectrometry apparatus 103 through a screen 102 having an aperture 102a. An ion emission mechanism 105 is arranged in the space inside the hollow vessel 101, that is, the ionization zone 104. The ion emission mechanism 105 is comprised of a lead wire 106 made of a refractory material such as tungsten and an ion emitter 107 comprised of for example alumina silicate doped with an alkali metal salt and attached to the lead wire 106. The lead wire 106 is led to the outside from an attachment part 108 provided at part of the hollow vessel 101. A gas introduction mechanism 109 is provided at the bottom 101a at the left end of the hollow vessel 101 in the figure. The gas introduction mechanism 109 introduces the target gas and one more gas which acts as a third-body in a mixed state to the ionization zone 104. The third-body gas is an inert gas such as nitrogen. It strips the excess energy occurring when metal ions attach to the gaseous molecules comprising the target gas (an analysis target substance without the metal ions) to make the ions of the sample gas and prevents the ions from again separating into the target substance and the metal ions.
The ionization zone 104 in the hollow vessel 101 is supplied with the third-body gas and target gas from the gas introduction mechanism 109 and evacuated by a vacuum pump (not shown) provided at the mass spectrometry apparatus 103 to be held in a pressure state of about 100 Pa. In that state, a current is run through the lead wire 106. Using the resistance heat, the ion emitter 107 is heated to about 600° C. When this happens, the substantially spherical ion emitter 107 produces metal ions on its surface. If a voltage of about 10V is applied to the lead wire 106 to give it a positive potential, the screen 102 is made the ground potential, and the inside of the mass spectrometry apparatus 103 is held at a negative potential, a potential gradient can be formed in the ionization zone 104. Due to this potential gradient, the metal ions are emitted from the ion emitter 107 and transported to the mass spectrometry apparatus 103 side. In the transport process at the ionization zone 104, the metal ions impact the target substance for analysis and gently attach to the charged locations thereof to thereby produce ions of the sample gas. The ions of the sample gas continue to be transported to the mass spectrometry apparatus 103 side as they are, pass through the aperture 102a provided at the screen 102, and are thereby emitted outside of the ionization apparatus.
According to the above ionization apparatus, when the target gas includes gaseous molecules comprised of an organic substance including weak bonds between atoms or a chemically active substance, if contacting the ion emitter 107, the target gas is broken down by the heat transmitted from the heated ion emitter 107 and polymerization or chemical reactions are caused. Due to these chemical reactions, substances other than the target substance for analysis which are originally not covered by the mass spectrometry are produced. The amount of production of these substances other than the target substance for analysis is extremely small and is substantially negligible in measurement by mass spectrometry. The substances other than the target substance for analysis, however, deposit on the ion emitter and other components facing the ionization zone and build up along with the measurement time (cumulative hours of operation of the apparatus). These substances deposited on the components are called “by-products” below. As a result of the buildup, the problems arise of a drop in the amount of emission of the metal ions, a detrimental effect on the transport of ions required for measurement, etc. and therefore a drop in the detection sensitivity.
An ion emitter 107 impaired in function of emission of metal ions sometimes can be restored in function by holding it in a vacuum or exposing it to oxygen in a heated state. This restoration is not always effective depending on the type of the target gas. Note that when the target gas includes an organic substance, this problem can be solved by the invention disclosed in Japanese Patent Application No. 2000-369876 of the same assignee as the present application.
For restoration of components facing the ionization zone 104, it is necessary to either stop the mass spectrometry apparatus or ionization apparatus and return the inside once to the atmospheric pressure and then clean it or remove the dirty components from the apparatus and clean them. If the mass spectrometry apparatus or ionization apparatus is stopped once to return the inside of the apparatus to atmospheric pressure, a long standby time will be required before it can be returned it to a state enabling measurement once again. Further, when the target gas is a silane (SiH4) based gas, solid substances (SiO2, etc.) will precipitate on the inside wall of the ionization apparatus due to the exposure to the atmosphere. These solid substances will enter into the mass spectrometry apparatus with its complicated and precise structure and make accurate measurement impossible.
Further, even when a substance other than the above organic substance or chemically active substance is used as the target gas, the gaseous molecules are adsorbed on to the wall inside the ionization apparatus. This state sometimes is held for a certain period of time and is a cause of a “memory effect”. In measurement by mass spectrometry without sufficient intervals provided between measurements, the target gas used in the previous measurement will remain in the ionization apparatus, though in a small amount, resulting in the problem of an inability of accurate measurement by mass spectrometry. This problem appears most remarkably when the target gas includes a substance highly adsorptive to solid substances etc.